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Date: Sat, 14 Apr 2012 05:37:12 +0400
From: Solar Designer <solar@...nwall.com>
To: john-dev@...ts.openwall.com
Subject: Re: JtR:Multi-GPU Setups

magnum, Samuele -

On Fri, Apr 13, 2012 at 11:22:30AM +0200, Samuele Giovanni Tonon wrote:
> On 04/13/2012 12:32 AM, magnum wrote:
> > I just created this wiki page. For now I just filled it with most of the
> > info from this thread, so it doesn't get lost. The list has become
> > fairly high-volume lately.
> 
> thanks magnum,
> you helped me breaking the momentum so i modified the page and now it
> should help a bit
> http://openwall.info/wiki/john/GPU-setup

Thank you!

FWIW, I've just removed the questionable statement of having dual GPUs
being non-useful yet.  It only applied to fast hashes.

For slow hashes, multiple instances of JtR are perfectly capable of
fully loading multiple GPUs in the same machine at once.  I experimented
with that the other day, got over 1.5M c/s combined speed at phpass with
incremental mode on my AMD + Nvidia cards.  This also kept two logical
CPUs busy.  With these two john processes reniced to -20 (as root),
I was also able to run a third john process with OMP_NUM_THREADS=6 (to
use the remaining 6 logical CPUs) without significantly slowing down the
GPU-using johns.  I measured up to 640W being drawn from the power
outlet with these things running at once.  (I'll need to improve cooling
before possibly running this for real - such as during the contest -
since my CPU got dangerously hot by the time I interrupted this test run.
The 7970 card was also extremely hot, but I think that's fine for it.
GTX 570 stayed much cooler, maybe due to its dual cooling fans.)

It does currently take an otherwise unused logical CPU per GPU for
optimal performance, though.  Otherwise the context switches on the CPU
reduce GPU load (since we're sending candidate passwords from the CPU,
and not in an async fashion with most formats).  (So with 8 GPUs in a
machine, I'd need to avoid any use of the CPU other than by the
GPU-using johns.)

Alexander

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